1. Field of the Invention
The present invention relates to a position measuring device for determining the absolute position, as well as a method for absolute position measuring 9.
2. Description of the Related Art
Absolute position measuring devices are increasingly employed in many fields, wherein the absolute position information is derived from a code track with code elements arranged one behind the other in the measuring direction. In this case the code elements are provided in a pseudo-random distribution, so that a defined number of sequential code elements constitutes a bit pattern. In the course of displacing the scanning device with respect to the code track by a single code element, a new bit pattern is already formed, and a sequence of different bit patterns is available over the entire measuring range to be absolutely detected.
Such a sequential code is called a chain code or a pseudo-random code.
It is stated in the publication “Absolute Position Measurement Using Optical Detection of Coded Patterns” by J. T. M. Stevenson and J. R. Jordan in Journal of Physics E/Scientific Instruments 21 (1988), No. 12, pp. 1140 to 1145, that each code element includes a predetermined sequence of two partial areas with optical properties which are complementary to each other.
Reference is made in the publication to GB 2 126 444 A. For generating the binary information in connection with such a Manchester coding, it is proposed there to compare the analog scanning signals of the code areas with a predetermined trigger threshold and to generate a binary information 0 or 1 as a function thereof.
This comparison with a fixedly predetermined trigger threshold has the disadvantage that fluctuations in the analog scanning signals can lead to an erroneous generation of the binary information.